A Passive Optical Network, PON, is a point-to-multipoint network architecture employing fibre cables from a central office to end-user premises. A PON can employ unpowered optical components such as splitters to enable a single optical fibre to serve multiple premises. A PON typically comprises an Optical Line Terminal, OLT, at the central office of the service provider and a remote node connecting a plurality of Optical Network Terminals, ONTs, which can be controlled and operated by end-users. A PON configuration reduces the amount of fibre and central office equipment required, as compared to point-to-point architectures. A passive optical network can be regarded as a form of fibre-optic access network.
FIG. 1 illustrates an example of a basic optical communication network, according to the prior art. In this example, a central office 100 comprises an OLT 100a which sends data signals on an optical feeder link 102 to a remote node 104 connecting a plurality of ONTs 106 by means of individual fibre links 108. A signal distributor 104a in remote node 104 receives the data signals on the feeder link 102 and sends each signal to a corresponding ONT 106 depending on the signal's wavelength. A component called “Arrayed Waveline Grating, AWG” may be used as signal distributor 104a in the Remote Node 104 for distributing the incoming data signals with different wavelengths over the different fibre links 108. Briefly described, the AWG is configured to route an optical signal received on an input port to a specific output port depending on the wavelength of the signal, which is a well-known technique in the art. Signals can also be transmitted over said links in the opposite direction, e.g. for signaling and data communication.
The optical feeder link 102 between central office 100 and remote node 104 in such networks is typically a single optical fibre used for transporting data signals of different dedicated wavelengths assigned to different ONTs. The feeder link 102 may be quite long, e.g. in the range of several kilometers, while the individual fibre links 108 connecting the ONTs 106 to the remote node 104 are typically shorter, typically less than one kilometer. An optical fibre in any of the links 102, 108 may be damaged at some point, for whatever reason, such that the faulty fibre causes a disturbance or break in the signal transmission. It is then naturally of interest to detect and find such a faulty fibre in order to repair the link accordingly with a minimum of impact to the network performance.
In order to supervise and monitor the performance of an optical network, Optical Time-Domain Reflectometry, OTDR, is typically used which is well-known in the field. Briefly described, an OTDR device at a central office injects a series of optical test pulses into the feeder fibre. The series of optical pulses, also called OTDR signal, travel through various network links towards the ONTs. Parts of the OTDR signals are reflected back towards the OTDR device. The back-reflected, or back-scattered, OTDR signal may be used for estimating the fibre's length and overall attenuation, including splitter losses.
A back-scattered and back-reflected OTDR signal may also be used to locate faults in fibres, such as breaks or bends, and to measure optical return loss. According to common language in this context, a “back-scattered” signal basically originates from any reflections along a fibre caused by a phenomena called Rayleigh Backscattering, whereas a “back-reflected” signal originates from a discrete event such as a fault in the fibre. In this description, the phrase “back-scattered and back-reflected” is thus used for signals caused by a faulty fibre. The above use of test signals can be initiated either at preset intervals or upon demand such as when an alarm function or the like is triggered by malfunction of the data transmission.
Generally, some requirements can be made on the performance of a monitoring or supervision system. The monitoring process should not influence regular data communication, i.e. it should be “non-invasive”. This is achievable by using a dedicated optical bandwidth for the test signals which is separate from the bandwidth used for data signals. Further, the technique should be sensitive to relatively low power fluctuations detectable in on-demand or periodic modes. Still further, the network supervision should not require any high initial investment. This mainly yields that no additional monitoring functionality should be needed on the ONT side, and a common monitoring functionality should be shared over a complete optical communication network or a group of networks.
However, the known solutions of today for network supervision or monitoring are generally deemed not to fulfill the above requirements. For example, most of the existing solutions significantly increase capital expenditures because they require either a customised OTDR device, which is expensive, wavelength specific components in the fibre links (drop links) towards the ONTs, causing power budget losses, advanced OLT transmitter upgrades, e.g. light path doubling, and so forth. Further, the existing solutions are often only capable of detecting a fault in a fibre link which introduces a significant loss of more than 5 dB, which is far above an expected and wanted threshold of typically 1 dB. Significant amounts of dedicated bandwidth is also required for the test signals to accomplish accurate supervision, which may not always be available e.g. when a great number of ONTs are connected to the network all using individual wavelengths for data signals.